1. Field of the Invention
The present invention relates to waveguides and fiber optics, and more specifically, it relates to techniques for combining the output of a fiber optic array to provide a coherent array of beams that are phase locked together.
2. Description of Related Art
The average power of fiber lasers has been increasing in the last few years due to the advent of xe2x80x9ccladding pumpingxe2x80x9d where the pump light is introduced into an inner cladding which surrounds the core, and which in turn is surrounded by a low index outer cladding. These devices have produced in excess of 100 Watts to date, but are ultimately limited in power by their small output aperture and the requirement that the fiber core only support one transverse mode.
Diode arrays have been phase locked together to increase their collective output power by employing a form of index guiding referred to as xe2x80x9cantiguiding.xe2x80x9d Index guiding is referred to as antiguiding when the refractive index is lowest in the regions aligned with the gain elements, and rises to a higher value or remains the same between elements. Since the light in the lower refractive index material leaks out of the lasing element regions, the term leaky-mode laser array is sometimes applied. A leaky-mode array is described in xe2x80x9cHigh-Power Leaky Mode Multiple-Stripe Laser,xe2x80x9d by D. E. Ackley and R. W. H. Englemann, Appl. Phys. Lett. 39(1), Jul. 1, 1981. A leaky-mode array is described in U.S. Pat. No. 4,348,763 titled xe2x80x9cMultiple Stripe Leaky Mode Laserxe2x80x9d incorporated herein by reference.
It would be desirable if a device utilizing the antiguiding technique could be used to combine the output of a fiber optic array. The present invention provides embodiments of such a device.
It is an object of the present invention to provide an antiguided fiber ribbon light amplifier and laser.
It is another object of the present invention to provide a scalable technique for generating kilowatts of power from a multicore phase locked aperture with high spatial and temporal coherence.
These and other objects will be apparent to those skilled in the art based on the disclosure herein.
The invention is a silica or other glass based fiber (such as fluoride or phosphate glass) that includes a core structure comprising an array of cores arranged in a spaced arrangement within a secondary waveguide region. Some of the cores are doped with a lasing impurity, and the remaining areas may be doped with refractive index-adjusting impurities. The doping of these adjacent regions is designed such as to promote xe2x80x9cantiguidingxe2x80x9d and xe2x80x9cleaky modesxe2x80x9d which provide for more robust single xe2x80x9csupermodexe2x80x9d operation. The ribbon laser may be pumped from the side with a laser diode bar, may be used as an oscillator or amplifier, and provides high power operation.
The fiber contains multiple doped cores in an array, with adjacent elements to keep the cores"" outputs coherently phased together. The cores can be arranged in a line, so as to be guided within a long aspect ratio rectangle, or ribbon. The multiple cores can be surrounded by an outer cladding that guides the pump light. One embodiment includes a cross-section of a 5-core ribbon, with a pump-guiding cladding structure surrounding the antiguided wave region. The antiguided region in this embodiment comprises Yb:silica and Ge:silica and the pump cladding comprises silica. Similar ribbon structures can be envisioned using phosphate glass as the base material. The ribbon is surrounded or embedded within a polymer coating.
In another embodiment, each core of 100 single mode cores is about 8 microns in diameter, with the cores uniformly distributed in a ribbon cladding structure that is 10 microns high by 2 mm wide. With this configuration, the optimum placement of laser diode bars is at a spacing of every 50 cm along the length of the ribbon for a core doping concentration of 1xc3x971020/cm3. The diodes can be coupled into the pump cladding using either prism couplers or diffraction gratings on the side of the ribbon structure. A five core embodiment is described and includes a low refractive index silica cladding to act as end mirrors to confine the wave laterally.
The proposed ribbon lasers can use a side-pumping scheme, with pumps placed periodically along the fiber. A laser diode bar is coupled into the outer cladding with a phase grating. Pump light from the diode bar propagates through the outer cladding to optically excite the laser impurity doped cores within the core structure. The core structure comprises laser impurity doped cores and index-increasing impurity doped cores. In an embodiment that utilizes the spaced diode pumping, a single fiber ribbon is wrapped around a pipe or mandrel. The ribbon includes laser diode bars periodically spaced along the length of the ribbon and water is flowed though the pipe or mandrel to provide cooling. One of the advantages of the ribbon structures proposed here is that they are very amenable to uniform thermal management. Because the heat generated in one gain loaded core exits in the ribbon structure without having to flow through another gain loaded core, all cores see the same temperature. Because cores at varying temperatures could impart varying phase differentials to the radiation traversing them, this is an important consideration in a device that is being designed to operate with a controlled phase across its aperture. For example, uniform temperatures would not exist across multiple gain loaded cores arranged in a two-dimensional pattern that filled an areaxe2x80x94a geometry that is presently popular with many of the so called photonic crystal fiber structures. For these devices, the cores in the center of the structure will be hotter than the cores in the perimeter of the structure due to their longer distance from any heat sink. One possible variation of the ribbon structure that is not a linear array, but nonetheless preserves the desirable property of having all gain cores see the same thermal environment, consists of a large circular pump cladding with gain cores located sequentially around the inside perimeter of the structure. If such a structure is cooled over its circumference, then all gain cores will see the same thermal environment.